1. Field of the Invention
The present invention relates to a charged-particle apparatus with a plurality of charged-particle beams. More particularly, it relates to an apparatus which employs plural charged-particle beams to simultaneously acquire images of plural scanned regions of an observed area on a sample surface. Hence, the apparatus can be used to inspect defects and/or particles on wafers/masks with high detection efficiency and high throughput in semiconductor manufacturing industry.
2. Description of the Prior Art
For manufacturing semiconductor IC chips, pattern defects and/or uninvited particles (residuals) inevitably appear on surfaces of wafers/masks during fabrication processes, which reduce the yield to a great degree. To meet the more and more advanced requirements on performance of IC chips, the patterns with smaller and smaller critical feature dimensions have been adopted. Accordingly, the conventional yield management tools with optical beam gradually become incompetent due to diffraction effect, and yield management tools with electron beam are more and more employed. Compared to a photon beam, an electron beam has a shorter wavelength and thereby possibly offering superior spatial resolution. Currently, the yield management tools with electron beam employ the principle of scanning electron microscope (SEM) with a single electron beam, which therefore can provide higher resolution but can not provide throughputs competent for mass production. Although the higher and higher beam currents can be used to increase the throughputs, the superior spatial resolutions will be fundamentally deteriorated by Coulomb Effect.
For mitigating the limitation on throughput, instead of using a single electron beam with a large current, a promising solution is to use a plurality of electron beams each with a small current. The plurality of electron beams forms a plurality of probe spots on one being-inspected or observed surface of a sample. For the sample surface, the plurality of probe spots can respectively and simultaneously scan a plurality of small scanned regions within a large observed area on the sample surface. The electrons of each probe spot generate secondary electrons from the sample surface where they land on. The secondary electrons comprise slow secondary electrons (energies ≤50 eV, simplified as SE for one and SEs for plurality) and backscattered electrons (energies close to landing energies of the electrons, simplified as BSE for one and BSEs for plurality). The secondary electrons from the plurality of small scanned regions can be respectively and simultaneously collected by a plurality of electron detectors. Consequently, the image of the large observed area including all of the small scanned regions can be obtained much faster than scanning the large observed area with a single beam.
The plurality of electron beams can be either from a plurality of electron sources respectively, or from a single electron source. For the former, the plurality of electron beams is usually focused onto and scans the plurality of small scanned regions by a plurality of columns respectively, and the secondary electrons from each scanned region are detected by one electron detector inside the corresponding column. The apparatus therefore is generally called as a multi-column apparatus. The plural columns can be either independent or share a multi-axis magnetic or electromagnetic-compound objective lens (such as U.S. Pat. No. 8,294,095). On the sample surface, the beam interval between two adjacent beams is usually as large as 30˜50 mm.
For the latter, a source-conversion unit is used to generate a plurality of parallel real or virtual images of the single electron source. Each image is formed by one part or beamlet of the primary electron beams generated by the single electron source, and therefore can be taken as one sub-source emitting the one beamlet. In this way, the single electron source is virtually changed into a plurality of sub-sources or a real or virtual multi-source array. Within the source-conversion unit, the beamlet intervals are at micro meter level so as to make more beamlets available, and hence the source-conversion unit can be made by semiconductor manufacturing process or MEMS (Micro Electro-Mechanical Systems) process. Naturally, one primary projection imaging system and one deflection scanning unit within one single column are used to project the plurality of parallel images onto and scan the plurality of small scanned regions respectively, and one secondary projection imaging system focuses the plurality of secondary electron beams therefrom to be respectively detected by a plurality of detection elements of one electron detection device inside the single column. The plurality of detection elements can be a plurality of electron detectors placed side by side or a plurality of pixels of one electron detector. The apparatus therefore is generally called as a multi-beam apparatus and the conventional yield management tool with a single electron beam is called as a single-beam apparatus.
Conventionally, the source-conversion unit comprises one image-forming means and one beamlet-forming means or one beamlet-limit means. The image-forming means basically comprises a plurality of image-forming elements, and each image-forming element can be a round lens or a deflector. The beamlet-forming means and the beamlet-limit means are respectively above and below the image-forming means and have a plurality of beam-limit openings. In one source-conversion unit with one beamlet-forming means, at first the plurality of beam-limit openings divides the primary electron beam into a plurality of beamlets, and then the plurality of image-forming elements (round lenses or deflectors) focuses or deflects the plurality of beamlets to form the plurality of parallel real or virtual images. U.S. Pat. Nos. 6,943,349, 7,244,949, 7,880,143 respectively propose an multi-beam apparatus with one source-conversion unit of this type. In one source-conversion unit with one beamlet-limit means, at first the plurality of image-forming elements (deflectors) deflects a plurality of beamlets of the primary electron beam to form the plurality of parallel virtual images, and then the plurality of beam-limit openings cuts off peripheral electrons of the plurality of beamlets respectively. The first cross reference proposes a multi-beam apparatus with one source-conversion unit of this type, as shown in FIG. 1.
In FIG. 1, the single electron source 101 on the primary optical axis 100_1 generates the primary electron beam 102 seemingly coming from the crossover 101s. The condenser lens 110 focuses the primary electron beam 102 and thereby forming an on-axis virtual image 101sv of the crossover 101s. The peripheral electrons of the primary electron beam 102 are cut off by the main opening of the main aperture plate 171. The source-conversion unit 120 comprises the micro-deflector array 122 with two micro-deflectors 122_2 and 122_3, and a beamlet-limit plate 121 with three beam-limit openings 121_1, 121_2 and 121_3, wherein the beam-limit opening 121_1 is aligned with the primary optical axis 100_1. If the beam-limit opening 121_1 is not aligned with the primary optical axis 100_1, there will be one more micro-deflector. The micro-deflectors 122_2 and 122_3 respectively deflect beamlets 102_2 and 102_3 of the primary electron beam 102, and thereby forming two off-axis virtual images 102_2v and 102_3v of the crossover 101s. The deflected beamlets 102_2 and 102_3 are perpendicularly incident onto the beamlet-limit plate 121. The beam-limit openings 121_1, 121_2 and 121_3 respectively cut off the peripheral electrons of the center beamlet 102_1 of the primary electron beam 102 and the deflected beamlets 102_2 and 102_3, and thereby limiting the currents thereof. The focusing power of the condenser lens 110 varies the current density of the primary electron beam 102, and therefore is able to change the currents of the beamlets 102_1˜102_3. Consequently, three parallel virtual images 101sv, 102_2v and 102_3v form one virtual multi-source array 101v with variable currents.
The primary projection imaging system 130 which comprises the transfer lens 133 and the objective lens 131, focuses the three beamlets 102_1˜102_3 to image the virtual multi-source array 101v onto the being-observed surface 7 and therefore form three probe spots 102_1s, 102_2s and 102_3s thereon. The transfer lens 133 focuses the three beamlets 102_1˜102_3 to perpendicularly land on the being-observed surface 7. The deflection scanning unit 132 deflects the three beamlets 102_1˜102_3 and consequently the three probe spots 102_1s˜102_3s scan three individual regions of the being-observed surface 7. Secondary electron beams 102_1se, 102_2se and 102_3se emitted from the three scanned regions are in passing focused by the objective lens 131, deflected by the beam separator 160 to travel along the secondary optical axis 150_1, and focused onto and kept within the three detection elements 140_1, 140_2 and 140_3 of the electron detection device 140 respectively by the secondary projection imaging system 150. Therefore each detection element will provide an image signal of one corresponding scanned region.
As critical dimensions are shrunk, smaller and smaller particles become killers in the yield. For inspecting particles, an electron beam has a relatively shorter wavelength (such as 0.027 nm/2 keV) compared to particle dimensions (down to several nm), and therefore can provide higher detection sensitivity for small particles than an optical beam. Higher detection efficiency comes from higher detection sensitivity, and higher detection sensitivity comes from higher image contrast. The conventional single-beam apparatus scans the being-observed surface with normal incidence of the primary electron beam, and is criticized in detection sensitivity for particle inspection.
To achieve high detection sensitivity, one dark-field electron-beam (e-beam) inspection method is proposed in the second cross reference, which employs an oblique illumination. As well known, when a primary electron beam is incident onto a surface of a sample with an incidence angle α (relative to the normal of the sample surface), the angular distribution of the SE emission conforms Lambert's law (proportional to cos ϕ, and ϕ is emission angle relative to the surface normal), and the angular distribution of the BSE emission comprises a diffusely scattered part with Lambert's distribution and a reflection-like part with emission maximum in the reflection direction. Obviously, in the case with an oblique illumination, the SE emission direction and the reflection-like BSE emission direction will be different from each other. Furthermore, if the sample surface with a particle is obliquely illuminated by the primary electron beam, due to the difference in normal direction, the sample surface and the particle will be different in both the SE emission direction and the reflection-like BSE emission direction. Please refer to FIG. 8A and FIG. 8B. For the primary electron beam (102_1), the SEs (102_1b1) and BSEs (102_1b2) from the sample surface (7) display a regular scattering situation and therefore become signal electrons for a bright-field image, while the SEs (102_1d1) and BSEs (102_1d2) from the particle (7_P) displays an irregular scattering situation and therefore become signal electrons for a dark-field image. The dark-field e-beam method employs the difference between the regular scattering on a sample surface and the irregular scattering on a particle thereon. A dark-field BSE imaging, which has a high image contrast due to the particle, can be obtained by specifically arranging oblique illumination, collection of BSEs and guiding SEs.
Accordingly, it is necessary to provide a multi-beam which can simultaneously obtain images of plural subareas of an area on a sample surface with high image contrast and high throughput. Especially, a multi-beam apparatus which can detect uninvited particles on wafers/masks with high detection sensitivity and high throughput is needed to match the roadmap of the semiconductor manufacturing industry.